The present invention concerns Rogowski coils. More particularly, the present invention concerns a printed circuit board based Rogowski coil and manner of making the same.
Rogowski coils are electrical devices which measure of magnetic fields and electrical currents. They have been researched over the past century and are well known to the scientific literature. Their origin traces to the invention circa 1912 of the Rogowski coil by W. Rogowski and W. Steinhousen. The device is useful for measuring electrical currents and operates on the basis of a magnetic field integration performed across a closed contour being equal to the current flowing through the contour. The coil provides a voltage output proportional to the time derivative of the current (di/dt), rather than a current output like other traditional current transformers (e.g., the iron core current transformer of FIG. 2).
Rogowski coils are popular because of their dynamic range and linearity. However, though theoretical requirements are known, manufacturers are still attempting to achieve a high quality coil that is both economical to manufacture and at the same time, capable of providing precise current measurements. The device (coil) should be insensitive to external influences, insensitive to the measured primary conductor position, and retain high precision (in the order of 0.3% or better) over its lifetime and across a wide temperature range (nominally −40 to 70 degrees Centigrade).
One known approach to making a Rogowski coil involves using a printed circuit. U.S. Pat. No. 5,414,400 describes a Rogowski coil implemented on a printed circuit plate provided with a circular cut-out. The coil is implemented by metal deposits on each of the two faces of the plate extending along radii, with electrical connections between the radii on one face and those on the opposite face being achieved via plated-through holes passing through the thickness of the plate. The patent fails to provide adequate means for external field cancellation due to inadequate handling of the coil return path.
U.S. Pat. No. 5,442,280 describes a method for manufacturing a printed circuit board-based Rogowski coil. The disclosed geometry provides very high turn density resulting in very high sensitivity. While high sensitivity is very desirable when measuring low frequency currents (50/60 Hz power system related), the patent fails to provide adequate means for external field cancellation. This problem is reported in U.S. Pat. No. 6,624,624 and is caused by inadequate handling of the coil return path.
A similar problem applies to the design reported in U.S. Pat. No. 6,313,623 (by Kojovic, et al.) in which two closely spaced coils with counter rotation are used to perform partial return path compensation.
Further attempts to design precision Rogowski coils are described in U.S. Pat. No. 6,624,624. Attempts to provide improved return path cancellation described in this patent results in significantly reduced coil densities, making the design less appropriate for low frequency (at about 50/60 Hz) current measurement applications. In addition, although improved, all reported geometries suffer from Z-axis (board thickness) related sensitivity effects with an error path normally offset in the direction of the Z-axis (board thickness).
J. D. Ramboz in “Machinable Rogowski Coil, Design and Calibration,” IEEE Transactions on Instrumentation and Measurement, Vol. 45, No. 2, (April 1996) pp 511–15 reviews traditional designs for Rogowski coils and discusses a “machinable” Rogowski coil constructed using machinable ceramic material to make a toroidal coil with a rectangular cross section. A thin, electrically conductive coating is then applied to the coil, totally encapsulating the ceramic core. Next, thin lines of the conductive material are removed by laser machining methods in a pattern which leaves coils as bands of conductive material located radially around the core. Each turn or band is connected to the next turn by a suitable indexing.
U.S. patent application Ser. No. 10/282,226 for “Current Transformer” describes a configuration whereupon a Rogowski Coil is implemented on a multilayer board. The patent provides a means for external field cancellation; however, a coil return path is centrally situated among the radial paths emanating from the aperture. Accordingly, this centrally situated return path necessitates at least a 4-layer printed circuit board, which must be manufactured at a higher cost.
An object of the present invention is to provide a precision printed circuit board Rogowski coil which provides excellent external field cancellation. It is further an object of this invention to provide for improved coil manufacturability, control of the coil resistance, compensation of the manufacturing variation of the coil gain, and temperature compensation.
These and other desired benefits of the preferred embodiments, including combinations of features thereof, of the invention will become apparent from the following description. It will be understood, however, that a process or arrangement could still appropriate the claimed invention without accomplishing each and every one of these desired benefits, including those gleaned from the following description. The appended claims, not these desired benefits, define the subject matter of the invention. Any and all benefits are derived from the multiple embodiments of the invention, not necessarily the invention in general.